Tokyo Tech discovery could aid organic electronics applications

A breakthrough by Tokyo-based scientists will enable the fine-tuning of phosphorus heterocycle materials’ electronic properties — a development that will be useful for applications such as the fabrication of organic electronics and hydrogen fluoride sensors.

The scientists have developed a new method of producing air-stable 1-aryl 1,3-diphosphacyclobutane-2,4-diyl materials — an open-shell singlet P-heterocyclic materials system — by direct arylation with electron-rich aromatic substituents. The ability to tune the physiochemical nature of the electron-donating 1-aryl 1,3-diphosphacyclobutane-2,4-diyls is expected to find applications including p-type semiconductors and sensors of hydrogen fluoride.

The Tokyo Institute of Technology group previously reported on a nucleophilic aromatic substitution (SNAr) process for producing air-stable 1-aryl 1,3-diphosphacyclobutane-2,4-diyls. However, this method only worked when electron-deficient N-heterocyclic aryl halides were employed as the electrophiles. Hence, there is still the need for new approaches for direct arylation of 1,3-diphosphacyclobuten-4-yl anions with electron-rich aromatic substituents that produce electron-donating and stable 1,3-diphosphacyclobutane-2,4-diyls for organic electronics.

The method developed by the Tokyo Tech group is based on the assumption that highly electrophilic arynes should react with and afford stable and electron-donating open-shell singlet P-heterocycles. In their experiments, the researchers generated arynes from the appropriate o-silyl triflates and fluoride and observed them reacting with 1,3-diphosphacyclobuten-4-yl anion under appropriate conditions, with the result being air-stable open-shell singlet P-heterocycles.

Notably, during the experiments the researchers also detected the presence of hydrogen fluoride (HF) by the P-arylated 1,3 diphosphacyclobutane-2,4-diyls. This release of HF resulted in remarkable changes in their photoabsorption properties. Analysis showed that 1,3 diphosphacyclobutane-2,4-diyls absorbed visible light at approximately 600 nm. Furthermore, the researchers fabricated a field effect transistor with a carrier mobility of 1.29 x 108 cm²/ Vs, an ON/OFF ratio of six and a threshold voltage of 4 V.

The Tokyo Tech group also found the optical absorption to show a ‘blue shift’ when the open-shell P-heterocyclic system ‘trapped’ hydrogen fluoride (HF); a finding that may enable the visual detection of the existence of HF and could be used for the development of HF sensors.

The researchers state: “In our study of HF trapping by the open-shell P-heterocyclic system, the electron-rich 1,3-diphosphacyclobutane-2,4-diyl system showed a remarkable blueshift of photoabsorption assigned as the transition from the HOMO to the LUMO, which enabled the identification of the presence of HF by visual observation.”

The research was outlined in the Angewandte Chemie International Edition.